WASHINGTON, Jan 16 (Reuters) - U.S. House Speaker Nancy Pelosi on Wednesday asked President Donald Trump to reschedule the annual State of the Union address until the federal government fully reopens, as a partial shutdown stretched into its 26th day with no signs of resolution.

With Trump's address scheduled for Jan. 29, Pelosi wrote him a letter citing security concerns because the Secret Service, which is required to provide security for the address, has not received funding during the impasse. The standoff was triggered by Trump's demand for a round of funding for his promised wall on the U.S.-Mexico border.

Senate Democrats planned to gather on the steps of the Capitol at noon EST (1500 GMT) on Wednesday to highlight the havoc the shutdown is wreaking, as 800,000 federal workers are furloughed or work without pay, and contractors do not receive payments.

Pelosi's letter to Trump pointed out that she had invited him to make the State of the Union address but said the shutdown complicated the situation.

"Sadly, given the security concerns and unless government re-opens this week, I suggest we work together to determine another suitable date after government has re-opened for this address or for you to consider delivering your State of the Union address in writing to the Congress," Pelosi wrote.

The State of the Union address is an annual speech at the Capitol in which the president a outlines the administration's goals for the year. All members of Congress and the Supreme Court attend, along with all but one member of the president's Cabinet.

The White House had no immediate comment on Pelosi's request and her letter appeared to take aides by surprise.

Representative Jim Jordan of the House Freedom Caucus, a group of conservative Republicans who are close allies of Trump, said Pelosi's move showed the lengths to which Democrats will go to obstruct Trump.

"It sure sounds like she's looking to not have the president come and give the State of the Union address, not have the commander-in-chief come and address the nation," Jordan told Reuters. "I think that just shows that they're more focused on stopping the president than they are on serving the country."

Speaking to reporters at the Capitol, Pelosi reiterated that security concerns had driven her request and suggested the president could deliver the address from the White House, according to CBS News.

Trump on Wednesday is expected to sign legislation that would ensure 800,000 federal employees will receive back pay when the government reopens.

Some government employees are being asked to return to work after being initially told to stay home during the shutdown, although they will not be paid on schedule.

Both the Internal Revenue Service and the Federal Aviation Administration on Tuesday said they would call back nearly 50,000 employees to process tax returns, refunds and other tasks or to work in aviation safety inspection.

The Washington Post reported that Food and Drug Administration workers also have been called to work without pay during the shutdown.

The shutdown began on Dec. 22 after Trump insisted he would not sign legislation funding the idled government agencies unless it included more than $5 billion for the border wall.

The wall was a signature campaign promise of his before the 2016 presidential election. Trump said at the time Mexico would pay for it but has since reversed himself, denying that he ever said Mexico would directly pay the bill.

Trump initially said he would take responsibility for the shutdown but later shifted blame to Democrats, who have controlled the House of Representatives since Jan. 3.

Trump needs the support of at least some Democrats in the Senate to pass funding legislation.

Economists have estimated that each week the shutdown continues will shave off 0.1 percent of economic growth.

More than half of Americans blame Trump for the government shutdown, according to a Reuters/Ipsos poll. A separate poll found the shutdown has affected four in 10 Americans, far beyond the federal employees directly feeling the impact of the funding lapses. (Additional reporting by Susan Cornwell, Susan Heavey, Steve Holland and David Morgan; Writing by Makini Brice and James Oliphant; Editing by Steve Orlofsky and Bill Trott)

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